The present invention relates to a control for two or more DC motors, in particular actuators driven by a DC motor for adjustment of furniture, such as tables, beds, chairs, etc, wherein the motors are connected to an adjustable power supply, and wherein the control is adapted to adjust the power supply to the individual motors such that the motors mutually keep the same or essentially the same speed independent of different loads on them.
Adjustment of beds and tables is performed using linear actuators of the type which comprises a spindle driven by a reversible DC motor, said spindle being provided with a spindle nut which, either directly or via an extension rod, is connected with the element to be adjusted. For a height-adjustable desk carried by a single leg at each side there is a linear actuator in each leg, and for beds there is one at each end for lifting the mattress frame. U.S. Pat. No. 5,224,429 Haworth Inc. describes such a height adjustable desk and EP 488 552 Huntleigh Technology plc describes a height adjustable bed.
The prior art for operating linear actuators to obtain uniform travel thereamong is based on a direct or indirect measurement of a distance travelled and a comparison of these. A typical approach is to count the number of revolutions of the spindle or another element in the transmission by an encoder or a rotary potentiometer, e.g. as described in the above US-patent. A comparison of the number of revolutions forms the basis for the control of the motors. It will be appreciated that a certain span of time elapses from a beginning aparallelism between the two actuators before a regulation can be carried out. These controls are relatively expensive and complicated since they are based on computer controls.